Almost all preschoolers are physically active at least part of the day. The problem is that preschoolers who don't get a chance to kick or throw a ball or jump and play hopscotch don't develop the basic skills they need to get involved in more organized sports and activities in grade school. As they get older, children drift into more sedentary behavior, depriving them of opportunities to build strong hearts and muscles and raising their risk of obesity and other chronic illnesses. Preventing childhood obesity could soon take a major "hop" forward with an app that teaches kids to do just that, and to skip, run and throw a ball. Fundamental motor skills like hopping, running and throwing a ball are critical to physical activity, and physical activity is a crucial part of preventing childhood obesity.